What's the difference between walk in and walk into in contexts like :
He walked into/in my room?
Are they interchangeable?
Sometimes they are used to mean the same thing, sometimes they aren't. It depends on the context.
"Into" gives the idea of transitioning from outside to inside.
"In" can mean the same, but not necessarily. For example, "He walked in your room" could mean that he was already in the room and he walked (no transition from outside to inside).
But if someone is outside a room and says "I'm walking in your room", they would mean that they're walking into the room. Whether that's incorrect or not, I don't know.
The main difference between walk in the garden and walk into the room is:
When used as noun, a walk-in customer is one without an appointment.
He walked into/in my room
Usually, when the verb walk like go and come expresses the movement of someone to the inside of a place, it takes the preposition into, not in. However, it's correct to use 'in' in the same sense as an adverb, without a following noun of place or space. For examples:
He walked in and sat down in a chair.
The door was open so I just walked in.