There's a particular kind of evening, the woodstove just getting going, the sky gone that early dark blue by five o'clock, when you sink onto the sofa and realize every pillow on it is either too flat to hold your head up or so overstuffed you're basically wrestling a bolster. That's the moment you start paying attention to pillows properly, not just as the thing that matches the throw.
Size and scale trip up more people than you'd guess, and I include myself in that, because for years I had a deep farmhouse sofa with a run of eighteen inch pillows on it that looked like crackers propped against a loaf of bread, totally dwarfed. The rule that actually works is to go up, not down, from what feels natural. A big sofa wants pillows in the twenty two to twenty four inch range, with a smaller one layered in front, and a single bed or armchair can handle something closer to eighteen. Too many small pillows crowded together reads fussy and a little desperate, while two or three generous ones, properly plumped, read like you know what you're doing. Where pillows fall down is anywhere they're doing a job better suited to something else, propping up your back for reading in bed, for instance. A feather insert sounds lovely until you're elbow deep in it at midnight trying to find enough support to hold a paperback, and that's when you want a firmer insert, not a decorative one, no matter how nice the cover is.
Feel matters more than people admit in a shop. A good down or down alternative insert should give that soft, almost exhaling sound when you drop onto it, not a crunch, and a cover with a bit of weight to the fabric, a heavier linen or a tight weave cotton, holds its shape through a season of guests flopping onto the sofa in a way that thin cotton just doesn't. For colors, the ones that live well over time are the muted ones, oatmeal, a faded rust, a deep moss, because bright white covers show every bit of woodsmoke and spilled cider by January, and I say that from direct experience with a cream pillow that did not survive hot cocoa season.
Where pillows go wrong is piling pattern on pattern until the sofa looks like it's arguing with itself, a busy plaid against a busy floral is a fight nobody wins, so pick one pattern to lead and let the rest be solid or textured. And steer clear of pillows with hard embellishment, sequins, heavy beading, anywhere someone might actually lean their head.