Luke Skywalker is a kind of virtuous village boy, living with his aunt and uncle on a remote planet. He has yearnings for something better but his loyalties are to his family - before the Empire's Stormtroopers come visiting.
Han Solo is a rough diamond. A streetwise adventurer with all sorts of dealings with all sorts of characters. Solo and Skywalker are an unlikely combination which is why they work well together in the film. It is easy to generate dramatic tension between two such differing characters. If all else fails drama can be created by giving the two characters the same goal. The obvious choice being the female lead.
Princess Leia is a curiosity. She is stroppy and not very useful so it is strange that the two male leads should both be interested in her. It helps the story no end but if considered for a moment it seems implausible. There's no accounting for taste. Cathy Gayle (played by Honor Blackman) and Emma Peel (played by Diana Rigg) from the Sixties 'The Avengers', Romana (played by Lalla Ward) in 'Doctor Who', Lisa Carol Fremont (played by Grace Kelly) in Hitchcock's 'Rear Window' (1954), and even Princess Aura (played by Priscilla Lawson) in the original Flash Gordon (1936), are a few notable ladies from a very long list that are both more feminine and useful to have around than Carrie Fisher's Leia.
Mark Hamill's career has been generally handicapped by his role as Luke Skywalker though he did well in 'Slipstream' (1989) playing a hard bounty hunter. Harrison Ford went on to take the lead in the Indiana Jones trilogy, Blade Runner and a number of other successful films.
Alec Guinness went on to
appear in the highly respected 'A Passage To India' (1984) (with Peggy
Aschcroft) and 'Kafka' (1991) completing one of the most brilliant careers
in film making. Guinness' triumphs include 'Great Expectations' (1946),
'Kind Hearts and Coronets' (1949), 'The Mudlark' (1950), 'The Lavender
Hill Mob' (1951), 'The Man In The White Suit' (1951), 'The Card' (1952),
'Father Brown' (1954), 'The Ladykillers' (1955), 'The Prisoner' (1955),
'The Bridge On The River Kwai' (1957), 'The Horse's Mouth' (1958), 'Tunes
Of Glory' (1960), 'Lawrence Of Arabia' (1962), 'Doctor Zhivago' (1965),
and 'Murder By Death' (1976).