1. A method comprising: a speech-analysis platform receiving speech data representing at least part of a speech session; the speech-analysis platform comparing the speech data with each of at least one stored speech-characteristics profile in an automated manner, wherein each such profile is associated with a person that is a target of surveillance, wherein the comparing step comprises using natural-language-processing (NLP) to compare the speech data with the at least one stored speech-characteristics profile on each of a lexical level, a syntactic level, a semantic level, and a discourse level, wherein said using of NLP does not comprise conducting voiceprint analysis; the speech-analysis platform determining in an automated manner from said using of NLP whether the speech data matches any of the profiles; and if the speech-analysis platform determines from said using of NLP that the speech data matches at least one of the profiles on each of the lexical level, the syntactic level, the semantic level, and the discourse level, the speech-analysis platform responsively storing an indication that the speech data matches at least one of the profiles.
5. The method of claim 1 , wherein the profiles comprise respective syntax-choice data, and wherein using NLP to compare the speech data with the profiles on the syntactic level comprises comparing at least one instance of syntax usage represented by the speech data with the respective syntax-choice data of at least one profile.