0 used before saying something that is not as important as the main subject of conversation, but is connected to it in some way: --
We had a marvellous meal at that restaurant you recommended - incidentally, I must give you the number of a similar one I know.
Incidentally, I wanted to have a word with you about your travel expenses.
1 used about something that is connected to the main subject but is not as important: --
We can complete it step by step however long the road is and it can't be completed however short the road is if you don't even mark your footprint.
Можем да го завършим стъпка по стъпка, колкото и дълъг да е пътят и не може да бъде завършен, колкото и кратък да е пътят, ако дори не маркирате отпечатъка си.