Panda dataframe Json flattening for all rows

I have below Panda Dataframe where there are three records.

I am able to do the JSON flattening using only one row at a time, but I try for all rows it doesn't work. JSON flattening for single row.

For all rows I need help as below code is not working at all for me

My Dataframe:

df = pd.DataFrame(
    columns=['data'],
    data=[
        [{ "answers_interface": { "__id": 1, "flags": {}, "heloc": 3 ,"balloon": 4 }}],
        [{ "answers_interface": { "__id": 2, "flags": {}, "heloc": 3, "balloon": 4 }}]
    ])

Convert df['data'] to list, then use json_normalize:

out = pd.json_normalize(df['data'].tolist())

Output:

   answers_interface.__id  answers_interface.heloc  answers_interface.balloon
0                       1                        3                          4
1                       2                        3                          4