One huge advantage of opera concerts over staged operas, is that they can easily be performed in smaller venues for a low budget. This is true of staged opera, for example Fulham Opera, but it is more unusual. The size of the performance space has an impact on how the character is portrayed.
When the audience is in close proximity, gestures and facial expressions can be less exaggerated than on a large stage, otherwise they could look insincere. For large, outdoor opera concerts, such as at Marble Hill Park Picnic Concerts, the characterisation (extent of expression and gestures etc) would need to be as large as for a big operatic stage. These are choices that a singer can make depending on the space available.