I'm not entirely sure how I do it but I get what you mean... I sort of get to know my characters by asking them questions about their life and make them answer what I already know about them. After a while, they start answering for themselves, in their tone and their vocabulary. Once they start doing that I just chat with them a bit before starting a scene they're in.
When I'm writing a scene it helps me to get into the mind of being a camera. I use blender and visualising myself as the little camera icon on blender that moves around can help. Then I sort of let the characters improvise over a loose throughline, like I'm watching a film. If it flows well, I rewind and play the scene over while writing it down this time, if not, I rewind and prod at it a bit to go in a different direction. I've got to the point where I can play and pause at will without getting out of the zone.
Generally this process is tiring so I have to have energy and usually in silence when I'm alone and my rabbit is sleeping. No distractions or parasite noise. Surprisingly I've gotten a lot of scenes to play out while doing light repetitive manual labour as part of my job (basically putting things in the right place when it's already in order for you, so you turn your brain off and muscle memory does the task for you). I find that being active like that while also having my brain free to imagine helps a lot.
Final tip is that if you're writing a scene later in the story, it helps to reread the scene directly preceding it (or the last one the characters were in) because it can remind you what they're feeling and how they talk. You're basically doing the job of a writer on a film set, reminding the actors where their head's at, what they've just been through, so rereading that can be the thin end of the wedge to get you fully immersed.