As a remote developer you need to understand two things: the person on the other end does not know who you are and you do not know who the other person is. Sure, you have met with them, even on video chats and all. But in remote you do not have everyday casual communication with that person (verbal or inverbal), you cannot see their habbits, their way of functioning in a regular environment. And they cannot see yours either. So - while you'd be yourself and communicate to a colleague who was sitting across from you in an office in your normal manner (casually, making jokes and so on), in a remote environment it is different and you need a bit more discipline.
Keep your tone formal
As I said, you do not know who is on the other end. The person on the other end might be a cool person not caring about rules and formalisms, but they do not have to. They could be a really meticulous grammar nazi pixel perfect freak. If it is the former, anything will work. However, if it is the latter, then a wrong tone in your "voice" can resonate quite loudly and make you seem unprofessional.
My advice is to keep it formal:
Don'tDo not use abbreviations.- Write full words and sentences - it takes you just as much time to write "Are you sure?" as it does to write "r u sure".
- Capitalize words where you need to - this applies to IM chats where people avoid hitting the Shift key for some reason.
- Avoid emoticons - the relationship between an employer and an employee is usually not one that would require emoticons.
It's funny how sarcasm can be no fun
Sarcasm? NO! Not in any form, shape or size. There are a few reasons for this, but the main one is that it just does not transfer well through electronic communication. If you think of communication as real life communication, video, audio, email, chat... the only one where sarcasm can work 100% is real life. In video and audio it is always possible that the person does not see or get your body language, or that the line breaks just as you make your punchline and then you leave them hanging... In chat and email it is tricky to understand what is sarcasm and what isn't. (e.g. did you ever receive an email from a client asking 50% off? Wasn't your first thought "Are you kidding me?")
If you get sarcasm right, you'll be the funny guy. Which is all nice and dandy, but not really required for the engagement. So - it gets you some personality points, let's quantify it to e.g. 10. But - if (for any of the many reasons!) you get sarcasm wrong, you can really insult the person on the other side. And - again - you do not know who is on the other side. So they might think "OK, Mario tried to be funny and it didn't work" or they might think "You're fired!". You very often do not know how the other side would react. So, getting sarcasm wrong could get you -1000 personality points. A bet where you can win 10, but lose 1000 is not necessarily a bet I want to take.
Conclusion / disclaimer
A few disclaimers here. The advice I am sharing is advice I learned through my engagements and that works for me. It doesn't have to be your cup of tea, don't hate me if it isn't. In my past I have also worked on setting up quite a few remote jobs, and this is what worked in most cases.
Also - it is possible that through the course of the engagement you will get to know the other person, or that they are really easy going and that the chats and calls will become informal very soon. If this happens, that's great - personally I also love working in an informal environment. But - don't take informal and easy going as a rule. Start formal and then take it down as you see fit.