I suggest you read this (AGS) wiki article:
http://www.americangirlscouts.org/agswiki/Giving_Personality_to_Characters
It's not going to answer your question directly necessarily, but I think it's a great place to start for your question.
As for my direct answer, I think the answer lies in the premise. How interesting the character is, or how fun the gameplay is, lies solely on what you start off with. To keep the gameplay interesting, first, you must have an interesting character, because all of the gameplay revolves around who and what the main character is.
In order to strike a balance, I suggest giving a little backstory, and have the rest of the game slowly delve into who this character is. A majority of what defines a characters personality is (in adventure games) how they interact with the environment. Simply because, that is what you'll be doing for about 90% of the game.
In order to define what an interesting character is, all you have to do is be a little twisted with your interpretation. Such as Bernard from Day of the Tentacle. He's a nerd, yes, but he's an interesting nerd, why? because of what he brings to the game. In this case, humour. Your main character should be your main source of comedy/action/horror and all those other feelings (unless you have more than 1 main character, then you'll be able to share it around) in how they perceive and react to the world around them.
I hope that helps.
http://www.americangirlscouts.org/agswiki/Giving_Personality_to_Characters
It's not going to answer your question directly necessarily, but I think it's a great place to start for your question.
As for my direct answer, I think the answer lies in the premise. How interesting the character is, or how fun the gameplay is, lies solely on what you start off with. To keep the gameplay interesting, first, you must have an interesting character, because all of the gameplay revolves around who and what the main character is.
In order to strike a balance, I suggest giving a little backstory, and have the rest of the game slowly delve into who this character is. A majority of what defines a characters personality is (in adventure games) how they interact with the environment. Simply because, that is what you'll be doing for about 90% of the game.
In order to define what an interesting character is, all you have to do is be a little twisted with your interpretation. Such as Bernard from Day of the Tentacle. He's a nerd, yes, but he's an interesting nerd, why? because of what he brings to the game. In this case, humour. Your main character should be your main source of comedy/action/horror and all those other feelings (unless you have more than 1 main character, then you'll be able to share it around) in how they perceive and react to the world around them.
I hope that helps.