Say I'd like to describe a situation where I naturally (not abruptly) join a conversation. I think the closest expression is "chipping in", but which preposition should I use? (Or do I need a preposition there at all?)
Bonus question: I also heard the idiom "slip in", as in "You should try to slip your thoughts in discussions". Can I use "slip in" in a similar way to "chip in" by not stating what to slip in? (e.g., "You should try to slip in discussions")
Best Answer
I'd use 'to'… but I'm not sure it ends there.
To chip in to [or into] a conversation is likely to be slightly abrupt, but not necessarily unwelcome.
There's a similar "chime in" which adds perhaps the hint of a bright & breezy addition to the conversation.
Neither of these alone specifically indicates you fully joined the conversation, you maybe just added one small contribution, but it doesn't necessarily preclude it.
You might differentiate further to explain a single interruption by using "chip/chime in with [information/comment.]"
To "slip into" a conversation is different. That implies your were already in the conversation but added some 'extra' information.
The potentially 'rude' interruption to join a conversation would be to "break in" or "butt in". This definitely carries the implication that your joining was not necessarily welcome, or appropriate.
Your offer of "You should try to slip in discussions" doesn't work, because there is the double meaning that you're talking whilst walking on ice… & fall.
You definitely shouldn't aim to "slip up" in conversations - that's to get something wrong, or make a mistake. Many of these apparent 'directions' in English phrases have specific meanings you cannot hope to be able to guess at.