You can check to see if there will be a problem - get a dildo and do a trial run in the bathroom.
If you want to take an enema, use only clean, warm water and experiment to find out how much you need to take. More water isn't always better, as it can bring stuff down from further up and require more enemas. The only reason you'd need more than about a cup would be if you're doing extreme things (deep penetration, fisting, etc.).
Porn stars take a risk for ATM. I'd imagine that they probably do some sort of prep similar to what they do for colonoscopies, with multiple enemas or that liquid stuff, but it's still a risk. If you're interested in it I'd strongly recommend condoms.