Did you know that R2-D2 is in Close Encounters? True fact. Dennis Muren, who worked on miniature and optical effects for the original Star Wars, also headed mother ship photography for Close Encounters. In one shot, Muren and Gregory Jein, the film's chief model maker, added a little R2 to the mother ship and illuminated it with backlight, so as not to be too obvious. If you're looking for the astrodroid hitchhiker, he appears at roughly the 1:54:00 time mark. (Source)

François Truffaut worked diligently on his English during production, but his French accent caused a few communication breakdowns. For example, when he delivered the line "They belong here more than we," some crewmembers heard, "Zey belong here, Mozambique." The phrase became a running joke on set, and t-shirts were even printed. Truffaut found the whole thing hilarious, which is good because we think Truffaut's English is very endearing. (Source)

It's not easy working with toddler actors. Spielberg came up with all sorts of ways to elicit three-year-old Cary Guffey's (Barry) natural performance. In the scene where Barry first sees the aliens, Spielberg had one crewmember dress as a clown and another in a gorilla suit and told them to stand off camera. When Guffey came around the corner and saw them, Spielberg caught the boy's natural surprise on camera. To get Guffey to smile, the gorilla removed its mask to reveal the crewmember's face. (Source)

To prevent the press from spoiling the film's ending, Spielberg kept the model of the mother ship locked up in his garage. (Source)