Acting is a skill that takes time to develop - time spent rehearsing and practicing. Not everyone can be an excellent actor, but a good acting ability is critical to portraying your role as Sephiroth. You'll want to focus on how you speak, how you stand, how you wield your sword, and your overall demeanor.
Remember, if you're going to act like me then you must remember that you are an typically an evil villian. For the purpose of this article, I will refer to myself mainly in third person. There are three versions of Sephiroth, though. First is Sephiroth before he discovered his past, both a hero among the people, and a SOLDIER for Shinra. Second, is insane Sephiroth. This is the guy who killed the President of Shinra, caused chaos, and chased after mother. Lastly is the final version of Sephiroth, Sephiroth after he accepts his destiny to rule. This Sephiroth is calm, cool, collect, and in-charge. You can pick which Sephiroth to act as, but often you will have to act as the final verson, because some outfits (KH outfits, for example) are from after you accept your fate.
1. Focus on your speech. Sephiroth usually has a deep, strong, clear voice. He does not stumble over words, and always thinks of his words in advance. Don't be afraid to speak slowly, find your words and then say them aloud.
2. Don't show much with your facial expressions. An overexagerated smile might be on your mind when at some exciting Anime Convention, but you can't do this. The only appropriate time to smile (especially teeth-bearing) is when about to defeat someone in battle.
3. Don't run, except when chasing or being chased. Sephiroth doesn't hasten, he will get there when he gets there. If you have need to zip around, perhaps you should not be wearing your outfit. Don't forget that black trenchcoats are usually fairly warm, and you might regret overexerting yourself. Besides, after running you may find yourself out of breath - and Sephiroth would never be caught out of breath.
4. Wield your Sephiroth sword properly. Never flail about with such a dangerous weapon, but just as important you should look skilled when you have it out. Random, sloppy chops are not good form.
