these tests are more unpleasant rather than painful. i never had the EMG, so can't tell.
anorectal manometry - you'll have to do a fleet enema before the test. they will ask you to squeez than relax. they check whether the pressure of your rectal and anal muscles are normal and whether the capacity/volume of your rectum is normal.
defecography - it's a video x ray of the rectum with contrast (barium). you'll have to do a fleet enema for this too. they give you barium to drink, and after i think half hour or so they call you in. they fill the rectum and vagina with thikcer barium, then you sit on a toilet like thing, the x ray machine is next to you and they ask you to squeez, then expel the barium like you do when you have a bm. the contrast enables them to see the anatomy and function abilities of the rectum.
marker test - each hospital/dr do it a bit differently. but for the most part, you are supposed to take a capsule that contains 24 little rings that are shown on an x ray. and every couple/few days they take an x ray and see how far they've gone. during the days of this test (which goes on at least for 5 days), you are not allowed to take any kind of laxatives or use enemas/suppositories - you must not do anything to help your bowels during these days, cuz they have to see their real functioning. some drs ask the patients to quit laxatives/enemas/suppositories etc few days before swollowing the capsule, but each dr works differently.