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Most actors fall into the trap of doing things the way they've seen them done already. Since nobody usually gets to hang out with friends while they take a crap, the only time you've seen it done is by an actor on-screen. Most dramas don't go near the bathroom so it's likely you saw this bowel movement in a comedy. Comedies often heighten and exaggerate for laughs and this will not help you drop a deuce credibly.

When you go poo you don't just groan and make contorted faces without motivation. Noises happen and facial expressions change, but there are physical factors at work that elicit these noises and faces and can help you make your shitting more organic.

Ab Tightening

It goes without saying that you need to push this one out. Otherwise you would sit, shit, get up and leave. To squeeze out these feces you need to flex the muscles around the bowels to help it along. This will be your abdominals, the same you use when doing situps. But instead of tightening for a second as you pull your body up to your knees as in a situp, this will be sustained flexing. You need to flex and hold those abs until you fill the toilet with your excrement. This is the source of most groans as you struggle to hold those muscles flexed.

Hold Breath, Sip Air

While tightening your abs you will find that most of your energy goes to this endeavor. Along with complete focus on this task, you will find it difficult to breath normally. The harder you are flexing your abs the harder it is to breath. Most people can't go long without breathing but we need to hold our abs to be able to convince this piece of poop to vacate. So you will find yourself mostly holding your breath with quick sips of air and slight ab releasing so you don't lose any headway you may have gained with earlier ab tightening.

Wince At Pain

Not only will this take a lot of patience and energy, it's often painful. This is where the facial contortions stem from. As you move a big piece of poop through your system it may stretch or even tear certain parts of your anatomy. This pain won't be chronic but rather in short, acute bursts. So you won't be in pain the whole time but rather just uncomfortable with moments of intense pain.

Releasing, Breathing Easy

The moment when you actually do defecate is when you can suddenly take it easy. You can breath normally and relax your abs. You may be panting at this time and may need to wipe the sweat from your brow. You may even exhibit a sigh of relief. You may sit back in your toilet and savor your accomplishment. If you are unlucky you will have to go right back to work, otherwise it will be time to get up and move on with your life.

Clearly this isn't a skill that will come in handy for dramatic actors. Some comedy directors may not find it funny enough. This isn't just about taking a dump, it's about portraying life from real life and not from media. Don't go to the first thing that comes to your head, take yourself through it and pay attention to the actions you take and you'll bring something personal and fresh to your portrayal.