The usages of 厚い and 篤い From what I read, is used when something is thick or abundant and is used when describing a serious illness. But what confuses me is that in a dictionary (Tagaini Jisho) both of these are in the same entry, is it because the kanji can be interchangeable? Is commonly used? And which kanji would be used for the second definition hospitable? The entry: Alternate writings: () 1. Thick, deep, heavy. [ () ] 2. Kind, cordial, hospitable, warm, faithful. 3. Serious (of an illness). [ () ] 4. Abundant. [ () ]

I guess you are referring to a dictionary entry like this one. I think that is not a reading (only is ), so that covers the meaning "serious illness", but is used often for this meaning. (Compare , meaning "serious illness".) I can't find the key for , but I'd guess that the little triangle means "beware, not ".

Note also that also means "kind", e.g. as in , so the meanings of and do overlap (but don't overlap with or ) and the choice of is partly a matter of personal taste. The dictionary provides an official guideline.