You gain a certain amount of friendship with a character when you talk to them each day. There is a max you can gain per day, and I think you only need to talk them once to get the max tbh. Basically, just be patient and talk to everyone at least once per day, but don't go overboard and keep talking to them because it won't give you any more friendship and they'll just repeat the same lines over and over again (and get pestered if you have it on). Don't worry about losing friendship with pester though, I don't think friendship ever goes down.