If you would like to make a comment or insert a remark in an ongoing conversation, it is polite to acknowledge what someone has just said before stating your own ideas.
That's interesting. I think that...
Interesting point. I would add...
Hmmm. I hadn't thought of that before.
Questions can also be a useful way of bringing new ideas into a conversation:
What do you think about . . .
Have you considered . . .
What about . . .
Sometimes a more direct approach is appropriate:
Can I add something here?
(Do you) mind if I interject something here?