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.

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.