Are "nie" and "niemals" always interchangeable?

Question

I always just assumed that "nie" and "niemals" were interchangeable, until my german level improved enough to start reading more complex articles and some literature and now I perceive a subtle difference.

Maybe it's an erroneous perception on my part, but it now seems like that there may be instances where they may not be interchangeable.

For example, some sentences like this seem relatively straightforward:

Ich habe nie im Leben ein grünes fliegendes Kaninchen gesehen

Ich habe niemals ein grünes fliegendes Kaninchen gesehen

but what about these following examples?

In meiner Kindheit bin ich nie von einer Biene gestochen worden. (could niemals also be used here? - it would seem strange to me...)

Ich habe nie Bonbons von einem Kind gestohlen

Ich habe niemals einen singenden Affen gestreichelt (here i'm not so certain..)

Willst du ins Cafe gehen? Niemals!

I'm sure the "mal" in "niemals" represents an element of repetition with respect to time but I can't put my finger on the difference with "nie".

Answer 1

In all your examples the words are fully synonyms in the sense that any change of meaning is very subtle. But there are some nuances.

1. "Ich habe nie im Leben ein grünes fliegendes Kaninchen gesehen." --- Nie im Leben is an idiom, niemals im Leben is not; it is a transparent and slightly stronger variant of the idiom.
2. "Ich habe niemals ein grünes fliegendes Kaninchen gesehen." --- Indeed, niemals here essentially plays the role of the stronger and more explicit nie im Leben or noch nie.
3. "In meiner Kindheit bin ich nie von einer Biene gestochen worden." --- See 6 for why in this context niemals would make me believe that this is an emphatic denial of a repeated accusation that the opposite is true.
4. "Ich habe nie Bonbons von einem Kind gestohlen." --- Here the words appear fully interchangeable to me.
5. "Ich habe niemals einen singenden Affen gestreichelt." --- Ditto.
6. "Willst du ins Cafe gehen? Niemals!" --- The phenomenon from 3 in pure form. This is an emphatic negation of the desire to go to the café now. You can use this even if you go to the café regularly and are not planning to change this habit. You just don't feel like doing it now. Replace niemals by nie, and it (technically) becomes a statement that you don't want to go to the café at any time.

While there is nothing particularly remarkable about 4 and 5 with either nie or niemals, I think native speakers would be much more likely to use noch nie. Even in 4, even though that seems to admit the possibility that this will change in the future. But it takes a conscious effort to withstand the temptation of the usual noch in such a sentence. Also note that whereas noch nie is extremely common, noch niemals is quite rare.