My experience is that even when waiting is the norm, you can still typically interrupt as long as you "ask permission" first (outside of really formal contexts where there's a rule against interrupting, anyway). Example of a less formal/explicit version: Raise hand slightly, go "Ah-" or "Uh...". Example of a more formal/explicit version: Interrupt with "Can I interrupt you for a second there?" The speaker can then either let you speak (e.g. "Yes?") or not. (Example of a less formal/explicit version: Raise hand in "stop sign" gesture or "just a minute" gesture, possibly also shake head. Example of a more formal/explicit version: "Hold on a moment, let me finish this.")

(This obviously overlaps some with Benquo's comment on metaconversational interruptions, though that seems to be about the reverse problem?)