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.

Some phrases

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?