The World of Henry Orient (1964)

In director George Roy Hill's coming-of-age drama-comedy, with a semi-autobiographical screenplay based on Nora Johnson's novel:

• the mischievous girlish-teenage, authentic-sounding friendship between two 14 year-old urban private school students: gifted and bright Valerie "Val" Boyd (Tippy Walker) and Marian 'Gil' Gilbert (Merrie Spaeth) who pranced with endless exhilaration and energy through many blocks of NYC, jumping while doing the splits ("splitzing") over garbage cans and fire hydrants
• their stalking, obsession and imaginative infatuation and idolization of eccentric, vain, self-centered, and mediocre concert pianist and lothario Henry Orient (Peter Sellers in his first US film, basing his performance on Oscar Levant), even in the first scene spying on him kissing skittish married housewife Stella Dunnworthy (Paula Prentiss) from the suburbs behind a large rock in Manhattan-NY's Central Park, and thwarting subsequent dalliances with her - while wearing conical Chinese peasant straw hats and tormenting Orient
• the contrast between the family life of the two girls -- the more stable and modest Marian with her kind, divorced mother Mrs. Avis Gilbert (Phyllis Thaxter) and understanding co-spinster "Aunt" Erica "Boothy" Booth (Bibi Osterwald), and the emotionally-disturbed, seemingly-unwanted Val (who regularly met with a psychiatrist), with wealthy parents living abroad who were busily engaged in international trade
• the scene of the two girls spying on Henry Orient (who feared the girls were spies sent by the wronged husband), and the devastating discovery that Val's rich, snobby and bitchy mother Isabel Boyd (Angela Lansbury) was also entrapped by the pianist's womanizing charms - and competitively turned her daughter's fantasies into reality
• understanding father Frank Boyd's (Tom Bosley) coming to his senses (and knowing Isabel's lying deception) in a bittersweet scene, and his comforting assurances to daughter Val that he would be divorcing Isabel to finally be a single father to her
• the final scene when the two girls, now a little older and more mature, met again and shared more advanced teenaged thoughts about boys, while comparing new hairdos, and applying lipstick