@R31GNI say generally its good form to give everyone a chance to post first, but sometimes it depends on who your interacting with at the moment, so if its just you and another character are just sitting in a room talking, its probably okay to not wait for everyone to post before continuing your conversation. However if there's big stuff for people to react to, its probably best to wait - depending on how people play.
See I like to play where each post incrementally adds something to the whole, so when I show up in the common room, the people who are there are already there and I also interact with an action (the pilot's FTL jump). So if something big happens, you react, someone else reacts, and then you react to their reaction, it means that people generally have to then write lots about their reactions to your reactions but without you ever noticing them cos you've already posted. That's why its better to wait. It makes stuff harder for other people and doesn't look good IMO if you dont.
Some people prefer to do stuff so that its like all posts are happening simultaneously and no one is just noticing what everyone else is doing, but I think this is a lazy and bad way to role play.