That's My Exact Opposite!

Sam
and
Lucifer

Jared played the main character of Supernatural, Sam Winchester .. He also played Lucifer, the archangel
(while possessing Sam's body) ..
There is no doubt that Sam and Lucifer are opposites of each other, Sam is a really innocent and kind man who dedicated his life for hunting the supernatural evil! and Lucifer is Lucifer ! He's Satan!

Bridget
and
Siobhan

Sarah played the twin sisters from Ringer, Bridget Kelly and Siobhan Martin .. Bridget and Siobhan are extremely different too, Bridget is a drug addict or was in the beginning .. Siobhan in the other hand, a rich new yorker ..

Elena
and
Katherine

In The Vampire Diaries, Nina starred portraying Elena Gilbert and Katherine Pierce .. Katherine is an old evil vampire, while Elena is a kind girl at least before season 3's finale! ..

Peter
and
Spider-man

Spider-Man is a movie starred by Tobey as Peter Parker and Spider-Man .. Those are different as well, Peter is a shy photographer while Spider-man is a superhero !

Jean
and
Phoenix

Famke played Jean Grey and Phoenix in X-Men: The Last Stand .. Jean is sweet woman who cares a lot for her friends which made her sacrifice herself for them in the second movie!, yet Phoenix reflexes all the evilness in Jeans mind and all of her "uncontrolled" powers!

Jared
and
Simon

The twin brothers in The Spiderwick Chronicles, Jared and Simon Grace, were played by Freddie .. Jared and Simon have opposite personalities. While Jared is the brave one, Simon is the more clever and rational.

Stanley
and
The Mask

Jim played both main characters in The Mask, Stanley Ipkiss and The Mask .. Stanley is a bank clerk that is an incredibly nice man, while The Mask is a cartoony romantic wild man!

Suggested by SFG¿mystic ⇊

Charlie
and
Hank

Jim played both main characters in Me, Myself & Irene, Charlie Baileygates and Hank Evans ..

Phoebe
and
Ursula

Lisa played both twin sisters in Friends, Phoebe and Ursula Buffay ..

Clark
and
Superman

Both of the main characters in Superman Returns, Clark Kent and Superman were played by Brandon..

King Louis
and
Philippe

He was the star of The Man in the Iron Mask as King Louis XIV and Philippe ..

Julius
and
Buddy

Both of the main characters in The Nutty Professor, Professor Julius Kelp and Buddy Love were played by Jerry ..

Julius
and
Buddy

As a remake of the above, Eddie played Professor Julius Kelp and Buddy Love in the 1996 version of the movie .. The Nutty Professor ..

Suggested by rackasheli