What is {cast member} of {Buffy the Vampire Slayer} that is {character role} is {Buffy Summers} ?
is {cast part} of {Buffy the Vampire Slayer} that's {character part} is {Buffy Summers} ?
is {cast member} of {Buffy the Vampire Slayer} that is {character role} is {Buffy Summers} ?
SELECT ?n WHERE { ?t property:cast_member ?n . ?t property:character_role resource:Buffy_Summers . resource:Buffy_the_Vampire_Slayer property:cast_member ?t }
select distinct ?obj  where {
?statement <http://www.w3.org/1999/02/22-rdf-syntax-ns#subject> <http://wikidata.dbpedia.org/resource/Q183513> .
?statement <http://www.w3.org/1999/02/22-rdf-syntax-ns#predicate> <http://www.wikidata.org/entity/P161> .
?statement <http://www.w3.org/1999/02/22-rdf-syntax-ns#object> ?obj .
?statement <http://www.wikidata.org/entity/P453> <http://wikidata.dbpedia.org/resource/Q1799459>
} 

SELECT ?obj WHERE { wd:Q183513 p:P161 ?s . ?s ps:P161 ?obj . ?s pq:P453 wd:Q1799459 }