A recent question on this site ("to suspect" vs "to be suspicious of") asks about the difference between "to suspect" and "to be suspicious of." An even more complicated situation involves when to use suspect as an adjective (as in "suspect reasoning" or "a suspect classification") and when to use suspicious (as in "suspicious thoughts" or "a suspicious detective"). Is there a general rule about when to use one adjective or the other? Are they always interchangeable? A somewhat similar question comes up in an older query titled "that things were suspicious".
In this connection I note Wilson Follett's comment (under transitive/intransitive) in Modern American Usage (1966): "Thus, suspicious should designate the persons harboring a suspicion and suspect the person who is the object of it. From this it follows that one cannot be seen carrying a suitcase in a suspicious manner." Presumably, Follett wants the author to use "in a suspect manner" here.
Follett's analysis is clearly prescriptive and just as clearly ignores centuries of frequent contrary usage; but is there any validity to his prescription as a way for writers to avoid possible ambiguity, as in the case of "suspicious behavior" versus "suspect behavior"?