Difference between "to foot the bill" and "to fill the bill"

I already knew the expression, "to foot the bill," and there is also, "to fill the bill." I initially thought this was just a variant, but on closer examination it seems to be an altogether different idiomatic expression.

So what's the difference?

Solution 1:

To foot the bill is to cover the costs or expenses of something or someone. One of the meanings of the verb foot is to pay for/cover. Thus, expressions such as I'll foot it, foot the expenses, foot the costs, etc, are also valid.

foot verb (informal): pay (the bill) for something, especially when the bill is considered large or unreasonable.

^{— Oxford Dictionaries}

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• Not to worry. I'll foot the bill.
• That wedding must have cost an arm and a leg! I wonder how they managed to foot the bill.

To fill the bill means to be suited/fulfill all requirements. (Fit may be more common than fill for this expression.)

fit (or fill) the bill: be suitable for a particular purpose

^{— Oxford Dictionaries}

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• We need someone who is aggressive, motivated and disciplined. Do you fit the bill?
• He doesn't look like someone who fills the bill.

Solution 2:

• fit (or fill) the bill: be suitable for a particular purpose (a partner is an ally or a companion, and you don't seem to fit the bill).
• foot (or pick up) the bill: (informal) pay the bill, especially when the bill is considered large or unreasonable.

^{form the New Oxford American Dictionary}