Aside from its interesting story, one factor that makes "Outlander" a hit TV series is the amazing chemistry between the show's lead stars. Sam Heughan and Caitriona Balfe look great together on-screen. In fact, many fans hope that they will be involved romantically off-screen as well.

According to Pop Sugar, the chemistry between both stars was evident during their first meeting. The pair immediately became good friends on the set of "Outlander." That friendship is their foundation to achieve a great performance in every scene of the long-running series.

First meeting

Sam, who plays Jamie Fraser, and Caitriona, who portrays Claire Randall, first met on the show's chemistry test which will determine if they can work out together as a pair.

It turned out that there is really something in them that made them click at once.

Caitriona said that she thinks it's quite funny during her first meeting with Heughan. She said he was the nicest guy and he puts her at ease. Although she felt a little nervous, her co-star was just solid and grounded. The two spent time together in London so they could get to know each other.

They talked about each other's lives while walking at the Hyde Park. They also shared their thoughts about their characters. Caitriona said that during their chemistry test, she knew it's going to be great since she found someone whom she could have a solid friendship with and she feels that Heughan is an amazing co-star.

The screen test

As for Sam, he said that he knew that Caitriona is the perfect actress for Claire during their screen test.

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It was a passionate scene wherein Jamie [VIDEO] was chastising Claire for running away but was caught by British soldiers. Sam said that she had her in a huge bear hug and Claire was very pissed off. When Caitriona left the screen test, they all knew that she is the one.

The two are great actors, and they could pull off every scene that they are asked to do. After the screen test, fans of the Starz series are so hooked up on the show and are enamored by the pair's amazing chemistry on-screen. No doubt, their chemistry is one important factor that makes the series a huge hit.

"Outlander" is a British-American television drama series which is now in its third season. It is based on a series of novels by Diana Gabaldon. Season 3 of the show is based on Gabaldon's third novel "Voyager, " and it will have 13 episodes. The fourth season is already confirmed which is based on the book, "Drums of Autumn."