In general, it is harder to write a smart character than a dumb character, because a smart character has to be capable of solving problems in smart ways. If you aren't careful and skilled, it's very easy to either end up with a "smart" character that's actually kind of dumb because he misses solutions and clues and details he should figure out (ex: many episodes of Doctor Who), or a "smart" character that's really just magic (ex: many versions of Sherlock Holmes, basically every scientist in superhero fiction. Also many episodes of Doctor Who).
To a certain degree. That said, there are ways you can fake it; the writer has hours or weeks to solve a problem and can then have a character divine that same solution in a few seconds, for example. You can use a computer to do complex calculations, then have the character perform them mentally. You can consult and research with people that are smarter or more knowledgeable than you, then have you character use their knowledge.
In general, it is harder to write a smart character than a dumb character, because a smart character has to be capable of solving problems in smart ways. If you aren't careful and skilled, it's very easy to either end up with a "smart" character that's actually kind of dumb because he misses solutions and clues and details he should figure out (ex: many episodes of Doctor Who), or a "smart" character that's really just magic (ex: many versions of Sherlock Holmes, basically every scientist in superhero fiction. Also many episodes of Doctor Who).
To a certain degree. That said, there are ways you can fake it; the writer has hours or weeks to solve a problem and can then have a character divine that same solution in a few seconds, for example. You can use a computer to do complex calculations, then have the character perform them mentally. You can consult and research with people that are smarter or more knowledgeable than you, then have you character use their knowledge.