The walls have been painted, the rugs rolled out and the furniture installed. So why doesn’t the living room feel finished? Because it isn’t.

What it needs are decorative accessories — pillows and throws, trays and bowls, stacks of books, maybe a plant or two — to make it look interesting and inviting. “A room before it’s accessorized is often flat,” said Alyssa Kapito, an interior designer in New York. “It doesn’t have the depth and character that only those elements can bring.”

The trick is finding the right accessories, which is harder than it looks. We asked interior designers for tips.

Make a Plan

The key to choosing accessories is understanding that they all need to work together. You can’t just select them one by one.

Think of them as a collection, Ms. Kapito suggested. “A home should be like a Gesamtkunstwerk,” she said. “Everything should go together. Everything should be thoughtful.”

That’s why accessorizing a living room involves more than merely buying whatever catches your eye. “We always think about accessories from the beginning,” said Kevin Dumais, an interior designer in Manhattan. “As we’re pulling the schemes together for different rooms, we look for fabrics that might amplify or accentuate the overall color palette, or further develop the mood we’re trying to evoke, and we hold onto those for later,” to use on pillows.

Shawn Henderson, a designer known for comfortable, pared-down interiors, said that before he buys any accessories, he figures out which ones are needed where. “I get very detailed about it, and draw items into the furniture plan,” Mr. Henderson said. “I draw a collection of things on each end table next to the sofa, and on the coffee table,” to study how the various items will work together.

Choose a Color Scheme

Accessories can either coordinate with the overall color scheme of a room, for a calm, cohesive look, or deliver punches of contrasting color and pattern to invigorate the space.

For an apartment in Manhattan’s West Village, for instance, Ms. Kapito used accessories in muted hues: pillows clad in a textured, off-white fabric from Holland & Sherry and a camel-colored cashmere throw, placed on a tan sofa, along with sculptural vases and containers in earth tones.

“It’s soft and easy on the eyes,” she said, but added a word of caution: “If you’re working with neutrals, getting as many different textures into the room as possible is important.”

Fawn Galli, a designer who worked on a house in Short Hills, N.J., took the opposite approach. She used subdued grays on the walls, sofa and rug, and then introduced bright pink in the stools and lamps, and acid yellow in a throw and piping on pillows. The goal was “to make it vibrant, electric and exciting,” she said. “It’s just a tiny bit of fabric, but the acid yellow steals the show, in a good way.”

Pick Your Pillows

Pillows and throws, which add visual interest and comfort, should never be an afterthought: The right pillows allow people to adjust their sitting position on a sofa, and a throw provides easy warmth.

How many pillows do you need? If you have a regular sofa, “a minimum of three and a maximum of five,” Mr. Dumais said, depending on how big it is. “On a sectional sofa, it could be more.”

They don’t have to match, or even be the same size. But they should be part of a coordinated plan.

For a family room in TriBeCa, Mr. Dumais used several pillows on a sectional sofa, in various sizes and materials, including felted wool, bouclé, alpaca, printed linen and fur, along with coral-colored, mohair-and-leather floor cushions. For the living room sofa in the same apartment, he said, “we brought in velvet, raw silk, leather and suede on the pillows,” again in various shapes and sizes, “just to create more interest.”

The combination may sound haphazard, but it wasn’t: All those seemingly mismatched pillows used materials, colors and patterns that related to each other visually to create a coherent composition.

Sometimes Mr. Dumais creates a sense of unity by choosing fabrics that reflect the surrounding environment, as he did for a home in Vero Beach, Fla. “It’s a beach residence, so we kept everything casual, light and easy,” he said, “with linens and cottons and things that are soft and cool to the touch. The pillows have stripes and beachy patterns.”

Add Sculptural Elements

Think like a curator and arrange favorite bowls, containers, candlesticks, vases and other objects on tables, shelves and the fireplace mantel. If they have sculptural appeal, it doesn’t matter if they’re empty.

“You want a wide assortment of things that add interesting shapes and extra layers to the room,” Mr. Henderson said, noting that “it’s always important to mix the materials.”

Describing the process of decorating a living room in an apartment near the High Line, in Manhattan, he said: “I have a stone coffee table, so I added wood and brass to warm it up,” in the form of a series of boxes. Then he added several other items to complement the boxes, including stacked books, a glass vase for flowers and a ceramic knot.

Ms. Kapito said she is always on the lookout for vintage pieces that can serve as distinctive accessories, especially unexpected things like a ceramic pitcher she placed on the mantel of a client’s home in Bellport, N.Y.

“Why have a pitcher on a fireplace? Because it’s beautiful,” she said.

Unusual accessories add a touch of whimsy, she said: “A pitcher filled with flowers is more playful than a vase.”

Combine Form and Function

Consider which accessories will make daily living a little easier — trays and catchalls that help contain clutter, for instance.

“Trays are accessory items that we use on almost every project, on almost every surface, because it’s somewhere you can collect all your things: your watch, your wallet, your phone, your keys,” Mr. Dumais said. “That way things aren’t just strewn about.”

Are you short on seating for children or occasional guests? Add a pair of lightweight stools, poufs or floor pillows that can easily be moved around.

Is your coffee table overwhelmed by magazines? Find an attractive basket or rack to hold them.

Do you have a fireplace? Look for distinctive tools and firewood containers that are pleasing to look at, even if you rarely build a fire.

Do you have a collection of favorite books? Use some of them as décor. “Books are actually a great way to add another color,” Ms. Kapito said, suggesting a small stacks of books that “are interesting to you, but also beautiful.”

Embrace Your Inner Gardener

Ever notice how homes in shelter magazines often have big bunches of cut flowers and branches on display, or spectacular potted plants? You can use the same strategy.

“To have that pop of green, that sign of life, is very comforting” in your home, Mr. Henderson said.

That could be as simple as placing a few succulents in small, sculptural pots. For something more dramatic, try adding large-scale plants to create a sense of verticality, even in a room with low ceilings.

“When a room needs some height, a plant is a really good way to bring your eye up,” said Ms. Kapito, who put a tall Dracaena reflexa tree in a corner of a West Village apartment. Just make sure that between the furniture and accessories, there are “different levels of scale,” she added, “so that not everything is on the same plane.”

Ms. Galli often uses the same strategy. “A tall palm tree can, in and of itself, add a sense of lushness and energy,” she said. “It’s only a plant, but it’s a game changer.”

Make a Few Adjustments

Even with advance planning, accessorizing a living room often requires small tweaks at the very end.

That’s why designers often take accessories out “on approval” from suppliers, Mr. Henderson said — so they can see what the items look like in place, before committing to buy them. If the accessories work well, the pieces are paid for and left where they are; if not, they are returned.

You can do something similar, as long as you’re careful about return policies: Buy more accessories than you think you’ll need, try them out, choose your favorites and return the rest.

As you survey your living room, look for areas that seem too empty or too contrived.

“There’s a moment at the end where it’s like using a highlighter to underline the whole design intent,” Ms. Galli said. “The accessories are conceived in the beginning, but in the end we ask, ‘Does it need to go further, or need to be pulled back?’ It’s that last moment of finding the balance and completing the story.”

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