It was a feature of the early stories being published that the gap between one adventure ending and the next starting be filled with bits and pieces to keep the audience interested. Probably the most famous was the staging of the arrival of Tintin by train when he returned from Russia, held in Brussels, and covered in the Petit Vingtième
We've also covered the sequences which took place before and after the trip to the Congo
The interview you talk of was in the weeks leading up to the start of Cigars
, and had the boy reporter being interviewed about his travel plans by JAM (the pen-name of Paul Jamin, who, along with Eugène Van Nyverseel, served as the earliest of Hergé's assistants).
I don't know anywhere where the interview is reproduced, off the top of my head, but it might be somewhere in a book - I dare say the collected stories which are being published by Casterman may include it.
Interestingly, it has been said that Hergé once asked E.P. Jacobs to choose his favourite of nearly 600 Petit Vingtième
covers, and Jacobs selected the atmospheric image on this issue - showing Tintin and Snowy peering out of the darkness from behind a statue of the Buddha, wreathed in smoke from an incense burner.