The key idea behind the word innocuous is safety. An innocuous home remedy, for instance, is safe and poses no danger of exacerbating whatever needs remedying in the first place. Innocuous is also used to mean inoffensive, as in the case of a remark that will be universally well received. On the flip side, sometimes this word suggests a little too much safety and not enough zest: innocuous is sometimes used to describe things that are not interesting, stimulating, or significant.
Something that is silly is absurd, ridiculous, or irrational—but not always in a negative way. Sometimes the word silly suggests amusement or even endearment (you silly goose!). Frivolous, on the other hand, almost always suggests a lack of value, merit, or necessity. A frivolous person is someone who is self-indulgently carefree and does not engage seriously with matters. A frivolous lawsuit is one that lacks legal merit and is not to be taken seriously.
Time is of the essence when it comes to these two adjectives, so we'll get right to it: quick describes a speedy tempo; expeditious emphasizes promptness and efficiency, especially in the completion of a task. An expeditious answer to an inquiry is one that takes no time at all. Expeditious is easy to confuse with expedient, but the latter (expedient) is better used to describe things that are suitable or advantageous for a specific purpose, such as politically expedient statement.