Paul Bettany portrays the ship's hard working doctor who is also the loyal friend and confidant of the Captain. As the Captain is far more of a passionate and romantic sort who pushes everyone to get the task at done. Crowe does this all very well, but what makes his performance really work is from the support of Bettany who acts as almost someone there just to bring the Captain down a bit. Bettany rightly does not portray the doctor in any way cynical but rather just a very down to earth man who is always very aware of what his job requires him to do.
Bettany finds just the right tone for the doctor to create the right dynamic with Crowe's Captain. Both show just the right warmth and kinship with one another, but they are not overbearing of it. In fact the two have such great chemistry here by simply playing that their friendship is pretty much just a given, it is not something that they even need to discuss. The two are both excellent in portraying the two have an easy understanding of each other, and they do this incredibly well because of how effortlessly the two create the relationship together.
In the first half of the film Bettany is very reserved in the film mostly going about the duties required of him. Bettany does this very well though as he shows the doctor as quiet but warming man. Bettany is particularly good in his scene where must amputee the forearm of one of the very young crew members. Bettany conveys well the reassurance the doctor is able to give the young boy, as he portrays the doctor as a man who has a genuinely good heart. Bettany is very good because he succeeds in creating the stress the doctor does suffer from his job, but still has the same reassurance.
The doctor's role drifts from being supportive as their mission seem far less likely of success despite the Captain's views to the contrary. The doctor frankly does his best to slam the Captain's thought of getting the ship no matter what, while the doctor seems the crew being discontented as well as his own personal regrets by not being to do research on the Galapagos island. Bettany is very strong in these scenes because he does not portray the doctor's anger in any sort of hatred toward the Captain, but rather an earned frustrations he feels over the Captain's selfish behavior. Bettany is able to portray properly that it is of Maturin's very real concerns for the crew, and his friend that propels his passionate disagreement with the Captain.
Bettany serves the needs of his part splendidly. It is one that is very supportive and he works the function brilliantly. When he needs to stand out on his own though he brings a power to the part very nicely as well. He particularly finds the inner strength of his character well. Bettany even succeeds in making the scene in which the doctor operates on himself after he has been accidentally shot. The scene very well could be one that would be hard to believe but Bettany stays absolutely convincing in the scene. Bettany's work here stand out in how natural he handles every scene, and every relationship within the ship. He stands out as he should, but he also stands almost to the side as he should. His work excels through how vivid he makes this doctor on this particular ship, he is not simply anyone he is Dr. Stephen Maturin.