Dr. Lanyon is one of the oldest friends in Dr. Jekyll's circle.
Some of the similarities that both characters share include the fact that they are both medical doctors who are highly respected in London, and each of them has his own views on science and how to approach their practices. They run in similar circles of wealthy, sophisticated, professional men who like the best things in life.
The doctor gave one of his pleasant dinners to some five or six old cronies, all intelligent reputable men, and all judges of good wine.
The key contrasts between Dr. Hastie Lanyon and Dr. Jekyll include that Lanyon, as opposed to Jekyll, is a believer of rational treatments and conventional medicine. He believes in prevention, as most physicians do, and he is quite hands-on with his methodology.
Jekyll, on the other hand, is more of an idealist. After all, isn't he busy preparing a weird concoction to separate his evil side from his good side? That's way against the typical mindset of any man of science.
Yet, there is additional evidence in the novel that shows that the biggest contrast between Jekyll and Lanyon has always been their approach to science. This is found in the letter that Jekyll writes to Lanyon in a cry for help.
DEAR LANYON, You are one of my oldest friends; and although we may have differed at times on scientific questions, I cannot remember, at least on my side, any break in our affection.
Alongside their differences in scientific opinion, we could also add that Lanyon's personality and physical appearance is different from Jekyll's.
Lanyon is described as ruddy, heavy, and with a brash demeanor.
He was a hearty, healthy, dapper, red-faced gentleman, with a shock of hair prematurely white, and a boisterous and decided manner.
Whereas Jekyll is described as decidedly "handsome," with a more peaceful nature and with a disposition for kindness.
He was a large, well-made, smooth-faced man of fifty, with something of a stylish cast perhaps, but every mark of capacity and kindness.
Still, notice that, even after parting ways with Jekyll, he saw Lanyon, of all people, as the only physician who could save him.
In the letter written by Jekyll, it is clear that Jekyll sees in Lanyon someone who was an equal in terms of ability and talents in the medical field.
While the two men argue about how things should be approached, in the end, the idealistic Jekyll falls prey to his own experiment and would obviously reach out to someone whom he would believe to be talented enough, or even more talented than him, to be able to save him from the huge disaster that he gets himself into.
Lanyon, my life, my honour my reason, are all at your mercy; if you fail me to-night, I am lost. You might suppose, after this preface, that I am going to ask you for something dishonorable to grant. Judge for yourself.
With these statements, we could also add that, between the two of them, Lanyon is also more mature and has more common sense than Jekyll.