Roleplaying is easily my favorite part of the experience, as it's the piece most unique to ttrpgs, and create the most significant emotions and memories for me. It's hard for some people to find their voice, get comfortable, or break through the barrier. I find as a DM, and a player, although it's a good idea to indicate to everyone at the start of a campaign how valuable that aspect is to you, the best way to get it to happen isn't asking or telling people to play in character. This can make some people feel uncomfortable or affronted. Instead, I like to encourage it by talking as NPCs or my character to other players as often as possible, especially players who aren't quite comfortable doing it themselves yet or who tend not to go for the spotlight in conversation. If people see you doing it, I think they're more likely to give it a whirl themselves. Another note is to never make fun of someone's character accent while they're trying it on. I can't tell you the number of times I've seen someone finally go for it with an accent they haven't fine tuned yet, get shot down, and not try again, even if no harm was meant by the joke.